Bromonapthalenes have no known industrial use or application, but have been identified as contaminants of Firemaster BP-6, the toxic mixture of polybrominated biphenyls used as a fire retardant and involved in a major episode of environmental posioning in Michigan. Structurally related to other halogenated aromatic xenobiotics, their toxicity and disposition seem to vary with the position of bromination. This work has studied the chemical disposition of 1,2,3,4,6,7-hexabromonapthalene (HBN), the most toxic bromonaphthalene in rats. The compound is incompletely absorbed after an oral dose. After iv treatment nearly 50% of the dose is excreted as metabolites within 3 days. However, the remainder of the dose seems to be extremely persistent, over 25% remaining in the liver after 35 days. It is possible that the compound spontaneously rearranged to its less toxic isomer, 1,2,3,5,6,7-HBN, which is also more resistant to oxidation. This suggests that what is being observed is the rapid metabolism and excretion of 1,2,3,4,6,7-HBN, while the rearrangement product persists.